An ethylene/tetrafluoroethylene copolymer (hereinafter sometimes referred to as ETFE) is excellent in heat resistance, chemical resistance, electrical insulating properties, flame retardancy, weather resistance and molding processability and thus is widely used as an insulating coating material for electric wires to be used for aircrafts, atomic power plants, automobiles or industrial robots, industrial tubes, tubes for fuel pipes, horticultural covering films, etc.
However, commercially available ETFE has a flexural modulus of elasticity of from 700 to 900 MPa, and the flexibility of such an ETFE is insufficient in applications required to have a flexibility such as tubes or films.
An ETFE is disclosed which is excellent in the flexibility and transparency wherein the molar ratio of repeating units based on tetrafluoroethylene (hereinafter referred to as TFE)/repeating units based on ethylene in ETFE is from 50/50 to 60/40, and the content of a fluorovinyl monomer which is copolymerizable to tetrafluoroethylene and ethylene is from 2 to 7 mol % (Patent Document 1). However, the flexural modulus of elasticity of ETFE described in Examples of Patent Document 1 is not low enough. Further, ETFE described in Comparative Examples has a low heat resistance.
An ETFE is proposed which has a content of repeating units based on TFE of from 62 to 90 mol % based on the total repeating units in ETFE and thereby has a low flexural modulus of elasticity and improved flame retardancy (Patent Document 2). However, an ETFE described in Example 3 of Patent Document 2 has a low melting point, an inferior heat resistance and a high flexural modulus of elasticity. An ETFE described in Example 6 has a low volume flow rate (flow value (mm3/sec)), and its molding property is inferior. Therefore, such an ETFE is impractical.